Method for controlling the supply and the discharge of hot air to and from, respectively, a blowing tunnel

ABSTRACT

A funnel-shaped arrangement (1) for drying and/or cooling vehicles or parts thereof. The funnel-shaped arrangement (1) is longitudinally divided into a number of sections each of which is provided with a plurality, preferably 60-250, of nozzles (7,20,21,22,23,24,25) which are substantially evenly distributed over the curved inner surface of the section and through which the gases are supplied and blown against the vehicle or the parts thereof passing through the arrangement (1), the pressure drop of the gases is measured across the nozzles (7,20,21,22,23,24,25) and the temperature of the gases before the nozzles (7,20,21,22,23,24,25). The measured pressure drop values are then compared with predetermined desired values corresponding to the prevailing temperature, whereupon a pressure increase or a pressure reduction signal is supplied to a first pressure changing means (29) for suplying the gases to the arrangement (1), depending on whether the measured values are below or above the desired values.

The present invention relates to a method for controlling the supply andthe discharge of hot and/or cold gases to and from, respectively, atunnel-shaped arrangement for drying and/or cooling vehicles or partsthereof, said arrangement being divided longitudinally into a number ofsections each of which is provided with a plurality, preferably 60-250,of nozzles which are substantially evenly distributed over the curvedinner surface of the section and through which the gases are suppliedand blown against the vehicle or parts thereof passing through saidarrangement.

In the car industry, there has been a switch over the past years towater-base paints, entailing certain problems in drying the paint on thevehicle parts. This is especially the case when a layer ofnon-water-base paint should be applied over a layer of water-base paint.

One problem of water-base paints is that these paints do not withstandsuch high temperatures as the paints previously used. This means thatsince it is desirable, for reasons of economy and efficiency, to use thesame drying units (blowing tunnels) and the same throughput rates asearlier, it will be necessary to supply a larger amount of hot air perunit of time than earlier, to compensate for the necessary reduction ofthe temperature of the hot air. This, of course, places increaseddemands on the system supplying and discharging the hot air to and fromthe blowing tunnel.

Another problem is that, in order to meet the increasing demands foroptimum energy utilisation, it is desirable to be able to control thesupply of heat to the different parts of the blowing tunnel, so thatthere is supplied precisely the amount of heat necessary for drying,e.g., the different parts of a car body placed in the blowing tunnel.

Yet another problem when using water-base paints is that they give offwater vapour when drying. This means that the exhaust air from theblowing tunnel will have a relatively high moisture content. If themoisture content inside the blowing tunnel becomes too high, the surfacelayer of the car body will dry very slowly. This means thatrecirculation of the exhaust air must be limited, although it isdesirable for better energy economy to recirculate as much hot exhaustair as possible.

A further problem encountered when using water-base paints is that ifthe moist air forming during the drying of the paint in the blowingtunnel would leak out into the surrounding atmosphere at the ends of theblowing tunnel, there would be a risk both of condensation and, thus, ofcorrosion on objects, such as spray booths, placed around the blowingtunnel, and of impaired function in the spray booth. Nor is it of coursedesirable from energy aspects that the hot drying air is emitteddirectly into the surrounding atmosphere without any previous energyexchange with cold supply air.

If the blowing tunnel is used for drying an outer layer of enamel on avehicle body, dangerous solvent vapours are instead emitted to thedrying air within the blowing tunnel. Nor is it of course desirable thatthese vapours are emitted into the surrounding atmosphere.

The primary object of the present invention therefore is to provide asolution ensuring efficient supply of heat to the different parts of theblowing tunnel.

According to the invention, this object is achieved by a method of thetype stated in the introduction to this specification, which ischaracterised by the steps of measuring the pressure drop of the gasesacross the nozzles, measuring the temperature of the gases before thenozzles, comparing the measured pressure drop values with desired valuescorresponding to the prevailing temperature, and supplying a pressureincrease or a pressure reduction signal to first pressure changing meansfor supplying the gases to said arrangement, depending on whether themeasured values are below or above said desired values.

The nozzles are suitably arranged in groups comprising a certain numberof nozzles, preferably 15-90, and the pressure drop and the temperatureof the gases can be measured, respectively, across and before eachnozzle group or each nozzle.

The pressure drops and the temperatures measured, respectively, acrossand before the nozzle groups or the nozzles are suitably compared withpredetermined desired values, whereupon such control signals aresupplied to valve means arranged in conduits provided between said firstpressure changing means and said nozzle groups or nozzles, that theopening degree of said valve means increases or decreases depending onwhether the measured values are below or above said desired values.

Another object of the invention within the scope of this solution is toensure that the hot drying air in the blowing tunnel and water- orsolvent vapours contained therein do not flow out at the ends of theblowing tunnel into the surrounding atmosphere.

This object is achieved in that the pressure differences between theinternal pressure of the arrangement and the pressure of the atmospheresurrounding the arrangement are measured, that the measured values arecompared with predetermined desired values, and that for adjusting saidpressure differences to said desired values, a pressure increase or apressure reduction signal is supplied to second pressure changing meansfor discharging the gases from the interior of said arrangement,depending on whether the measured values are below or above said desiredvalues.

Yet another object of the invention within the scope of this solution isto ensure that the air humidity within the blowing tunnel does notbecome too high while at the same time the exhaust air from the blowingtunnel is recirculated in a manner acceptable from energy aspects.

This object is achieved in that the moisture content of the gases ismeasured after said first pressure changing means, that the measuredvalue is compared with a predetermined desired value, and that such acontrol signal is supplied to valve means arranged in a recirculationconduit provided between the suction side of said first pressurechanging means and the delivery side of said second pressure changingmeans, that the opening degree of said valve means increases ordecreases depending on whether the measured value is below and abovesaid desired value.

The pressure changing means preferably consist of fans, and the speedand/or blade angles thereof are suitably modified as a function of saidpressure increase and pressure reduction signals.

The invention will be described in more detail hereinbelow withreference to the accompanying drawings.

FIG. 1 schematically illustrates a blowing tunnel for drying vehicleparts, such as car bodies, the supply and discharge of hot air to andfrom the tunnel being controlled according to the method of the presentinvention; and

FIG. 2 is a principle diagram of a control system for carrying out themethod according to the present invention.

The blowing tunnel 1 shown in FIG. 1 has a top part 2, a bottom part 3,and two opposite side walls 4, 5.

The top part 2 consists of a planar top plate 2' provided externallywith three blow boxes 6 and internally with nozzles 7 (see FIG. 2). Thenozzles 7 are divided into groups, the nozzles in one and the same groupcommunicating with a single blow box 6 via registering openings 8 and 9provided, respectively, in the top plate 2' and in the side of the blowbox facing the top plate 2'. Each group of nozzles comprises 30-90nozzles, preferably 60.

The bottom part 3 consists of a bottom plate 3' and two side strips 10and 11 connecting the bottom plate 3' to the side walls 4 and 5,respectively. Parallel to the side strip 11, the bottom plate isprovided with a rail 12 serving as a guide rail for the left-hand pairof wheels of a transport truck (not shown). The truck is designed fortransporting e.g. a newly-painted car body through the blowing tunnel 1in the direction of the arrow F, the enamel on the car body being driedby the hot air in the blowing tunnel. The transport truck is pulledthrough the blowing tunnel by means of a chain 13.

The side walls 4 and 5 each consist of three planar side plates 4a, 4b,4c, and 5a, 5b, 5c, respectively, which are joined longitudinally toeach other and to the top plate 2' and the side strips 10 and 11 of thebottom plate 3' such that the cross-section of the blowing tunnel 1 hassubstantially the same shape as the cross-section of an ordinary carbody. This means that the distance between the inner sides of the sideplates and of the top plate and a car body located in the blowing tunnelwill be approximately the same throughout the entire blowing tunnel.

The side plates 4a, 4b, 4c and 5a, 5b, 5c are each provided, like thetop plate 2', externally with three blow boxes 14, 15, 16 and 17, 18,19, respectively, and internally with nozzles 20, 21, 22 and 23, 24, 25,respectively (see FIG. 2). The nozzles are divided into groups (see FIG.1), the nozzles in one and the same group communicating with a singleblow box. This takes place through registering openings provided in theside plates and in the sides of the blow boxes facing the side plates.Each group of nozzles comprises 15-40 nozzles, preferably 30 (the nozzlegroups of blow boxes 16 and 19), 36 (the nozzles groups of blow boxes 15and 18) and 24 (the nozzle groups of blow boxes 14 and 17).

Further, the blowing tunnel 1 is divided longitudinally by means of fourdeflectors 26 provided along its inner periphery, into three sectionswhich can be supplied with different flows of heat depending on thedesired drying process. When a car body is located in the blowingtunnel, the deflectors 26 will cover about half the width of the spacebetween the inner sides of the side plates and the outer side of the carbody, whereby the deflectors can thus reduce the exchange of heatbetween the different sections. Since a deflector has also been providedat each of the ends of the blowing tunnel, the deflectors will alsoreduce the emission of heating into the atmosphere surrounding theblowing tunnel.

The blowing tunnel 1 is further provided with through ducts 27 forexhaust air which are so provided in the top plate 2' as to extendtransversely on each side of each blow box 6 and the group of nozzlesassociated therewith. The exhaust ducts 27 open into a suction box (notshown) arranged around all the blow boxes 6 on the outer side of the topplate 2'.

As appears from FIG. 2, illustrating only the central one of the threesections of the blowing tunnel 1, the blow boxes 6; 14; 15; 16; 17; 18and 19 are connected by conduits 28, 28a, 28ab; 28a², 28a² b; 28a³, 28a³b; 28a⁴ ; 28b, 28ba; 28b², 28b² a and 28b³, respectively, to a fan 29for supplying hot air to the interior of the blowing tunnel 1 throughthe nozzles 7; 20; 21; 22; 23; 24 and 25. Further, the fan 29 isconnected by a conduit 30 to an air preheater 31 connected in turn, by aconduit 32, to a heat exchanger 33.

The conduit 28 of course also branches off to the other twoblowing-tunnel sections disposed on each side of the section shown inthe Figure.

Further, the above-mentioned suction box is connected, by a conduit 34,to a fan 35 for discharging the air supplied to the interior of theblowing tunnel. The fan 35 is connected, by a conduit 36, to the heatexchanger 33 and, by a recirculation conduit 37, to the air preheater 31via the conduit 32.

From FIG. 2 also appears that the conduits directly communicating withthe blow boxes, i.e. conduits 28ab, 28a² b, 28a³ b, 28a⁴, 28ba, 28b² aand 28b³, are provided with throttles 38, 39, 40, 41, 42, 43 and 44,respectively. The recirculation conduit 37 is also provided with athrottle 45.

To control the opening degree of these throttles, as well as the speedof the fans, the drying installation described above is provided with acontrol system which will be described in more detail hereinbelow inconnection with a description of the mode of operation of the dryinginstallation.

Air from the atmosphere surrounding the drying installation is suppliedto the heat exchanger 33 by means of a fan (not shown). This supply airis heated in the heat exchanger by that part of the exhaust airdischarged from the interior of the blowing tunnel 1 which is notrecirculated to the air preheater 31 via the recirculation conduit 37.From the heat exchanger the supply air, now partially heated, isconducted to the air preheater 31 where it is mixed with therecirculated exhaust air. This mixture of supply and exhaust air isheated in the air preheater to the desired temperature, whereupon it ispassed to the fan 29 via the conduit 30.

The temperature to which the mixture is heated of course depends on whattype of paint or enamel or other surface layer should be dried in theblowing tunnel, but it generally is in the range of 40°-250° C.,preferably 50°-80° C.

The fan 29 supplies the air now heated to the blow boxes through theconduits associated therewith, The way in which the supplied air isdistributed between the different blow boxes is determined by theopening degree of the throttles in the above-mentioned conduits. Theopening degree of each throttle 38, 39, 40, 41, 42, 43 and 44 isadjusted by a control unit 46, 47, 48, 49, 50, 51 and 52, respectively,receiving measuring signals from a pressure drop sensor 53, 54, 55, 56,57, 58 and 59, respectively, and a temperature sensor 60, 61, 62, 63,64, 65 and 66, respectively. Each pressure drop sensor measures, for theair supplied to the associated blow box at the prevailing opening degreeof the throttle concerned, the static pressure drop across the group ofnozzles pertaining to the blow box, while the corresponding temperaturesensor measures the temperature of this air before the blow box, butafter the pertaining throttle.

Each control unit compares the measured pressure drop value with adesired value corresponding to the prevailing temperature. If themeasured value is below the desired value, such a control signal issupplied to the pertaining throttle as to increase its opening degree.However, if the measured value is above the desired value, such acontrol signal is instead supplied to the throttle as to decrease itsopening degree. If the measured value corresponds to the desired value,no control signal is supplied to the throttle.

Since the size of the flow of heat supplied by a certain group ofnozzles is determined both by the temperature (measured by thetemperature sensor) of the air passing through the group of nozzles, andby the flow of this air, in turn determined by the density of the air,the cross-sectional outlet area of the nozzles and the air velocity,determined by the static pressure difference (measured by the pressuredrop sensor) across the group of nozzles, it is actually the flow ofheat of the group of nozzles that is adjusted by means of the controlunit for the throttle pertaining to the nozzle group. The size of theflow of heat for the air passing through a certain control unit thus isdetermined by the desired values of the associated control unit.

These desired values may, for instance, be changed from a control panel(not shown).

From this control panel it is also possible to act on the control unitsso as to supply a closing signal to the throttles. This may be usefule.g. in cases where only certain parts of the car body have beenrepainted. In this case, closing signals are supplied to all throttleswhose associated nozzle groups are so disposed that, in operation, theywould blow hot air onto non-repainted parts of the car body.

The total flow of air supplied to the blow boxes by the fan 29 isdetermined by the speed of the fan. The speed of the fan is adjusted bya control unit 67 receiving measuring signals from the above-describedpressure drop sensors and temperature sensors (see FIG. 2). In thecontrol unit 67, the pressure drop values measured by the sensors arecompared in a conventional fashion with the desired values correspondingto the prevailing temperature. If the majority of the measured valuesare below the corresponding desired values, the control unit will supplysuch a control signal to the fan as to increase its speed. If, on theother hand, the majority of the measured values are above thecorresponding desired values, the control unit will supply such acontrol signal to the fan as to decrease its speed. If the majority ofthe measured values correspond to the desired values, no control signalis supplied to the fan.

When an enameled or painted car body is being dried in the blowingtunnel, solvent vapours and water vapour, respectively, are emitted. Forhealth reasons, these solvent vapours must not escape at the ends of theblowing tunnel into the surrounding atmosphere and, therefore, anegative pressure must be maintained in the blowing tunnel. Nor is itdesirable from energy aspects that the hot drying air flows out into thesurrounding atmosphere at the ends of the blowing tunnel (see theintroduction to this specification). Also, since it is not desirablethat unheated air from the surrounding atmosphere flows into the blowingtunnel at the ends thereof to cool the drying air in the tunnel, thisnegative pressure should thus be kept as small as possible.

In order to maintain such a pressure balance in the blowing tunnel, thepressure difference between the pressure in the blowing tunnel and thepressure of the surrounding atmosphere is continuously measured by meansof a pressure sensor 68. This sensor supplies a measuring signal to acontrol unit 69 comparing the measured value with a predetermineddesired value which is slightly below zero.

If the measured value is below the desired value, the control unit 69supplies such a control signal to the fan 35 as to decrease its speed,whereby the flow of air discharged by the fan from the interior of theblowing tunnel 1 through the exhaust ducts 27, the suction box and theconduit 34 decreases, i.e. the pressure in the blowing tunnel increases.If the measured value is above the desired value, the control unit 69supplies such a control signal to the fan 35 as to increase its speed,whereby the flow of air discharged from the interior of the blowingtunnel 1 increases, i.e. the pressure in the blowing tunnel decreases.If the measured value corresponds to the desired value, no controlsignal is supplied to the fan.

The fan 35 supplies part of the exhaust air discharged from the interiorof the blowing tunnel, to the heat exchanger 31 via the recirculationconduit 37 and the remainder to the heat exchanger 33, from which theexhaust air, now cooled, is thereafter emitted into the surroundingatmosphere.

As pointed out above, a certain amount of water vapour is emitted to theair inside the blowing tunnel when drying water-base paints, which meansan increase of the moisture content of the exhaust air. Since thesurface layer of the car body dries slowly at too high a moisturecontent, this means that it is necessary to reduce the amount of exhaustair that is recirculated, despite the fact that the exhaust air has ahigher energy content than the supply air delivered by the heatexchanger 33.

The amount of exhaust air to be recirculated is adjusted by means of acontrol unit 70 receiving a measuring signal from a moisture sensor 71.Since it is the moisture content of the air supplied to the blowingtunnel that is of importance, the moisture sensor 71 is so positioned asto measure the moisture content of the air after the fan, but before theconduit 28 branches off to the different blow boxes.

The control unit thereafter compares the measured value with apredetermined desired value. This desired value should be less than 0.03kg water/kg air, preferably less than 0.02 kg water/kg air. If themeasured value is below the desired value, the control unit suppliessuch a control signal to the throttle 45 of the recirculation conduit 37as to increase its opening degree.

If the measured value is above the desired value, the control unitsupplies such a control signal to the throttle 45 as to decrease itsopening degree. If the measured value corresponds to the desired value,no control signal is supplied to the throttle. If the supply air has amoisture content exceeding the abovementioned desired value, it ispreferably conducted through a dehumidifier (not shown) before beingconducted into the heat exchanger 33.

The invention is of course not restricted to the embodiment describedabove, but may be modified in several different ways within the scope ofthe accompanying claims. For instance, the control units of the fans mayadjust the blade angles of the fans instead of the speed of the fans, orthese control units may instead control the fans indirectly by adjustingthe opening degree of throttles or guide vanes provided before or behindthe fans.

In the embodiment described above, the blowing tunnel has been suppliedwith hot air only, but it may of course also be supplied with cold airfor cooling the car body before a new surface layer is to be applied toit. In such a case, air from the surrounding atmosphere is conducteddirectly into the fan 29 without passing through the heat exchanger 33and the air preheater 31. If particularly cold air is required, the aircan be conducted through an air cooler before being supplied to the fan.

The different sections of the blowing tunnel may of course be suppliedwith air that has been heated and/or cooled to different temperatures,the section through which the car body passes first then being generallysupplied with the hottest air.

In order to prevent solvent vapours formed in the blowing tunnel fromleaking out from the blowing tunnel at the ends thereof, it is possible,in addition to the above-mentioned negative pressure, also to create airlocks at the ends of the blowing tunnel. The air supplied to the airlocks often is a mixture of hot and cold air taken directly from thesurrounding atmosphere. The air locks of course also prevent the hot airat the ends of the blowing tunnel from escaping into the surroundingatmosphere. This also ensures appropriate supply of heat to and, thus,adequate drying of the surface layer which has been applied to the frontand rear portions of the car body The negative pressure in the blowingtunnel of course also prevents hot air and solvent vapours from leavingthe blowing tunnel.

According to the embodiment described above, the pressure drop and thetemperature are measured, respectively, across and before each group ofnozzles, but if it is desirable to obtain a more accurate adjustment,the pressure drop and the temperature may of course be measured,respectively, across and before each nozzle.

I claim:
 1. A method for controlling the supply and the discharge ofgases to and from, respectively, a funnel-shaped arrangement (1) fortreating equipment, said arrangement being divided longitudinally into anumber of sections each of which is provided with a plurality of nozzles(7,20,21,22,23,24,25) which are substantially evenly distributed overthe curved inner surface of the section and through which the gases aresupplied and blown against the equipment passing through saidarrangement, characterized by the steps of measuring the pressure dropof the gases across the nozzles (7,20,21,22,23,24,25), measuring thetemperature of the gases before the nozzles, comparing the measuredpressure drop values with desired values corresponding to the prevailingtemperature, and supplying a pressure signal to first pressure changingmeans (29) for varying as required the supply of gases to saidarrangement (1).
 2. Method as claimed in claim 1, characterized in thatthe nozzles (7,20,21,22,23,24,25) are arranged in groups comprising acertain number of nozzles, and in that the pressure drop and thetemperature of the gases are measured, respectively, across and beforeeach group of nozzles.
 3. Method as claimed in claim 1, characterized inthat the pressure drop and the temperature of the gases are measured,respectively, across and before each nozzle (7,20,21,22,23,24,25). 4.Method as claimed in claim 1, characterized in that the pressure dropsand the temperatures measured are compared with predetermined desiredvalues, and in response to such comparison control signals are suppliedto valve means (38,39,40,41,42,43,44) arranged in conduits (28ab,28a²b,28a³ b,28a⁴,28ba,28b² a,28b³) such that the opening degree of saidvalve means is modified as required in response to the receipt therebyof such control signals.
 5. Method as claimed in claim 1, characterizedin that the pressure differences between the internal pressure of thearrangement (1) and the pressure of the atmosphere surrounding saidarrangement are measured, in that the measured values are compared withpredetermined desired values, and in that for adjusting said pressuredifferences to said desired values a corrective pressure signal issupplied to second pressure changing means (35) for discharging thegases from the interior of said arrangement (1) as required.
 6. Methodas claimed in claim 1, characterized in that the moisture content of thegases is measured after said first pressure changing means (29), in thatthe measured value is compared with a predetermined desired value, andin response to such comparison a control signal is supplied as requiredto valve means (45) arranged in a recirculation conduit (37) providedbetween the suction side of said first pressure changing means (29) andthe delivery side of a second pressure changing means (35) such that theopening degree of said valve means is modified as required in responseto the receipt thereby of such a control signal.
 7. Method as claimed inclaim 6, characterized in that said pressure changing means are fans(29,35) and in that the operating conditions of the fans (29,35) aremodified as a function of the control signal received thereby.